As a first step to studying the function of extremely large myofibrillar proteins using molecular genetics approaches, genes encoding these proteins must be isolated and cloned. Due to several advantages in the use of Drosophila for genetic manipulations when compared with other organisms, we attempted to identify the gene encoding a homologue of vertebrate nebulin in Drosophila. Several attempts utilizing library screening and polymerase chain reaction technologies failed to identify a gene having sequence similarity to human nebulin. These results suggest that, if a functional analog of nebulin does exist in Drosophila, its nucleotide and amino acid sequence is likely to be quite different from that found in vertebrates. In an attempt to isolate a clone carrying large portions of the coding sequence for mouse nebulin, a cDNA library was constructed using conditions designed to optimize the chances of cloning very large cDNAs. Several putative nebulin cDNA clones have been isolated from this library, and we are currently in the process of characterizing them. Mutations of the cardiac myosin heavy chain gene are a major cause of familial hypertrophic (FHC), a serious genetic disease of the heart. By studying skeletal muscle fibers which utilize the same form of myosin as found in the heart, we have determined that missense mutations in the myosin heavy chain gene sometimes result in abnormal myofiber mechanics, but that isometric force output need not be affected to cause FHC.